W3 Total cache tries to create a dropdown list of S3 Buckets to which the IMA account has access. While listBucket is a valid IAM policy to list the contents of an S3 bucketS3, listBuckets isn’t valid. The correct policy reference is ListAllMyBuckets.

But this only covers the policy to list the S3 Buckets. If you’re using W3 Total Cache with Cloudfront, you also need to be able to list the Cloudfront distributions:
{
"Version": "2014-10-31",
"Statement":[{
"Effect":"Allow",
"Action":["cloudfront:ListDistributions"],
"Resource":"*"
}
]
}

From a security perspective, it’s not ideal that the IAM policy has to be able to list the distributions and buckets. A better solution would be to provide a field in which to manually enter the bucket or distribution name.